Operating system: Darwin-18.7.0-x86_64-i386-64bit 64 Bits
Graphics card: AMD Radeon Pro 580 OpenGL Engine ATI Technologies Inc. 4.1 ATI-2.11.20
Broken: version: 2.81 (sub 16), branch: master, commit date: 2019-12-04 11:32, hash: rBf1aa4d18d49d
Short description of error
Decimate's Un-Subdivide mode no longer works.
I used to use Un-Subdivide a lot in Blender 2.8, but it has stopped working in 2.81.
Exact steps for others to reproduce the error
Create a plane.
Subdivide it a few times.
Add the Decimate modifier.
Switch to Un-Subdivide mode.
Notice no effect.